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Formation of the ‘Great Unconformity’ as a trigger for the Cambrian explosion

Abstract

The transition between the Proterozoic and Phanerozoic eons, beginning 542 million years (Myr) ago, is distinguished by the diversification of multicellular animals and by their acquisition of mineralized skeletons during the Cambrian period1. Considerable progress has been made in documenting and more precisely correlating biotic patterns in the Neoproterozoic–Cambrian fossil record with geochemical and physical environmental perturbations2,3,4,5, but the mechanisms responsible for those perturbations remain uncertain1,2. Here we use new stratigraphic and geochemical data to show that early Palaeozoic marine sediments deposited approximately 540–480 Myr ago record both an expansion in the area of shallow epicontinental seas and anomalous patterns of chemical sedimentation that are indicative of increased oceanic alkalinity and enhanced chemical weathering of continental crust. These geochemical conditions were caused by a protracted period of widespread continental denudation during the Neoproterozoic followed by extensive physical reworking of soil, regolith and basement rock during the first continental-scale marine transgression of the Phanerozoic. The resultant globally occurring stratigraphic surface, which in most regions separates continental crystalline basement rock from much younger Cambrian shallow marine sedimentary deposits, is known as the Great Unconformity6. Although Darwin and others have interpreted this widespread hiatus in sedimentation on the continents as a failure of the geologic record, this palaeogeomorphic surface represents a unique physical environmental boundary condition that affected seawater chemistry during a time of profound expansion of shallow marine habitats. Thus, the formation of the Great Unconformity may have been an environmental trigger for the evolution of biomineralization and the ‘Cambrian explosion’ of ecologic and taxonomic diversity following the Neoproterozoic emergence of animals.

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Figure 1: Sauk Sequence in North America.
Figure 2: Phanerozoic sedimentation patterns in North America.
Figure 3: Middle to Late Cambrian palaeoenvironments and sedimentology.
Figure 4: Summary of major tectonic, geochemical and sedimentary patterns over the past 900 Myr.

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Acknowledgements

We thank D. Canfield, P. Cohen, W. Fischer, S. Finnegan, N. Heim, A. Carroll and R. Dott for discussion, and N. Butterfield, M. Foote, E. Hammarlund, P. Myrow, B. Wilkinson, R. Wood, S. Holland for feedback on ideas. Fieldwork and analysis was aided by P. Burke, J. B. Caron, L. Curtin, F. Dwan, Z. Feng, P. Fenton, L. Finley-Blasi, X. Hou, J. Lackey, C. Qi, J. Peng, J. Tian, J. Vorhies, Y. Yang, X. Zhang and Y. Zhao. Work was supported by NSF EAR-0819931 (to S.E.P.) and EAR-1046233 and DUE-0942447 (to R.R.G.).

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S.E.P. contributed Macrostrat-derived data, R.R.G. contributed sample-derived data. Both authors contributed to the development of ideas and writing.

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Correspondence to Shanan E. Peters.

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The authors declare no competing financial interests.

Additional information

Data for aspects of this analysis derive from Macrostrat (http://macrostrat.org).

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This file contains Supplementary Figures 1-8, Supplementary Tables 1-3 and additional references. (PDF 9479 kb)

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Peters, S., Gaines, R. Formation of the ‘Great Unconformity’ as a trigger for the Cambrian explosion. Nature 484, 363–366 (2012). https://doi.org/10.1038/nature10969

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